This invention relates in general to surface acoustic wave devices and in particular to surface acoustic wave devices for use with high temperature gases.
Surface Acoustic Wave (SAW) devices are electronic components that generate guided acoustic waves along a surface of the device. As the acoustic waves propagate along the surface of the device, any changes to the characteristics of the propagation path affect the velocity, and/or the delay, and/or the amplitude of the surface wave. Changes in the wave velocity and or delay can be monitored by measuring changes in the frequency or phase characteristics of the device while changes in amplitude are measured directly from the device. The changes in frequency, phase, and/or amplitude are then correlated to a physical quantity, such as pressure or temperature for measurement, or the detection of the presence of a specific gas. Thus, the device may be used as a sensor. Additionally, SAW devices also may be used as delay lines and resonators in electronic systems, which may be required to operate in harsh environments such as exposure to high temperature gases.
SAW sensors are among the most sensitive and widely used physical and chemical sensors in liquid and gas environments because the propagating acoustic wave has its energy concentrated close to the device surface. Along an arbitrary surface wave propagation direction, a particle in the substrate material describes an elliptical trajectory, with displacement components normal and parallel to the device surface. For liquid sensor applications, any SAW device operational mode with a significant particle displacement component normal to the surface suffers severe attenuation, thus compromising the device performance. However, this is less of a concern for gas sensor applications, since gases generally do not excessively absorb the wave energy. Accordingly, a regular, or generalized, SAW operational mode may be used for gas sensor applications.
SAW devices are typically fabricated on single crystal anisotropic substrates that are also piezoelectric, such as quartz. A piezoelectric material produces electrical charges when the material is subjected to stress. Furthermore, the phenomenon is reversible. A SAW device used as a sensor to measure temperatures, pressures, or the presence of a gas, typically includes a pair of spaced apart intertwined aluminum interdigital electrodes disposed upon the surface of the substrate. Each of the interdigital electrode sets forms a transducer. One of the transducers creates mechanical stress within the substrate by applying an electric field to the crystal. The electric field is oscillatory to create a mechanical wave. Thus, the transducer converts the electrical signals applied to the device into the electromechanical surface acoustic waves that are propagated along the surface of the substrate. The other transducer converts the received mechanical wave back into an electric signal for comparison to the original signal.
As an example of the application of SAW devices, one of the changeable characteristics of the propagation path is the temperature of the surrounding medium, which may be either gas or liquid in nature. The surface waves velocities, which are determined by the orientation, or cut, of the crystalline material used to fabricate the sensor, and the type of crystalline material used to fabricate the sensor, are temperature dependent. Thus, it is possible to correlate the SAW device change in surface wave velocity and material expansion along that orientation to the ambient temperature of the gases or liquids surrounding the device.
High temperature gas sensors are of interest for the aerospace industry as a safety tool for detection of fuel leaks in jet engines, early fire detection and detection of a hostile environment. High temperature gas sensors also are needed to increase combustion efficiency of jet engines, thereby reducing travel costs and air pollution due to unburned jet fuel. While Surface Acoustic Wave (SAW) devices have been successfully used in the past to measure gas temperatures, quartz, a widely used substrate for such devices undergoes an α to β transition at about 570° C. and loses its piezoelectric properties. Additionally, aluminum, the most commonly used material to form the interdigital transducers for a SAW device becomes soft when the temperature exceeds a few hundred degrees Centigrade and actually melts at 660° C. Thus, it is apparent that known SAW temperature sensors are limited as to temperature range and cannot be utilized to measure high temperatures, such as temperatures in excess of a few hundred degrees Centigrade. Accordingly, it would be desirable to provide a SAW gas sensor that could be operated at high temperatures that are beyond a few hundred degrees Centigrade.